Move to pooled channels and a streaming AppleScript shell

[ryanwhitney]

Major change to the core of how Control communicates with apps.

Previously:

• Open new channels for each command.
• Send individual actions, follow with status scripts, parse response.
• Send a heartbeat after each action to ensure we're still connected.

After this change, things are much more complicated:

• Open dedicated channels that stay open: osascript -s s -l AppleScript -i
• Open streaming interactive shells, parsing out responses as we go.

With dedicated channels and streaming shells, we hugely reduce the overhead in opening/closing channels and starting up osascript processes. This saves tiny—but significant—amounts of time, shaving 50–300ms per actions. This is especially noticeable when pressing the play/pause button which switches state based on server response or updating the volume slider.